Long and thin-wall cylinder are widely used in the machinery industry for the cylinder walls of various types of storage tanks. Its quality is extremely important to the safety requirements. In view of the cross-section structural stress and seam length during splicing welding,it is difficult to control the welding quality or finish the inner surface. This paper presents a special welding machine design for longitudinal seam welding of long and thin-wall cylinder. By integrating the new beam-pressing and the new arm-locking structure,the elastic deformation of the steel plate during the bending and clamping can be counteract to ensure the welding accuracy.Based on single-sided welding and double-sided forming technology,the existing cylinder welding process flow and inner-side forming control are optimized to improve the welding efficiency and quality. Experiments show the design of this special machine meets the welding requirements of long and thin-wall cylinder.